The production of di-or polyisocyanates such as tolylene diisocyanate (TDI) by phosgenation of tolylene diamine (TDA) and the subsequent distillative purification of the crude isocyanate, i.e. of the TDI, are common knowledge. Common to all of the known processes for distillative purification of the crude TDI is that the distillation affords not only the desired purified TDI but also higher-boiling components which, as a minimum, need to be made suitable for sending for proper disposal. The prior art for treatment of so-called distillation residues from TDI production describes various processes. General aims for residue treatment are maximization of the TDI yield, minimization of the amount of residue generated and a very useful, cost-effective and simple recycling of the residue amount no longer usable in the TDI production process. The workup of residues from isocyanate production is of increasing economic interest since the amount of residue and the amount of material of value present therein increases with increasing plant size.
The thermally induced undesired formation of higher polymers from diisocyanates by reaction with traces of moisture, amines and with one another to form, for example, ureas, uretdiones, biurets, isocyanurates, carbodiimides and uretonimines is common knowledge and often described in the literature. Substantial disadvantages of these undesired side reactions are that the formation thereof consumes material of value (TDI) and that they result in uncontrollable polymer growth which is accompanied by an increase in viscosity. This is the case in particular for residue concentrations in excess of >10% in TDI. In many cases the polymer growth likewise results in compounds that are insoluble in organic solvents. Such residues are thus convertible into solutions easily handleable in terms of process engineering only at comparatively greater effort and in low concentrations. Only a small number of costly and inconvenient processes have been brought to bear for such residues due to the poor handleability thereof. The economic efficiency of the workup of such residues is thus further significantly reduced.
To minimize the isocyanate yield losses the distillation residue may be transferred into a stirred and heated container and mixed with high-boiling hydrocarbons, preferably bitumen, inert under the distillation conditions to distill-off the free isocyanate still present in the residue as completely as possible (EP 0 548 685 A2). The remaining residue freed of isocyanate may be discharged as a free-flowing solid and sent for incineration. Disadvantages of this process include not only the use of a substance foreign to the process (bitumen) but also yield losses due to polymerization of the isocyanate since the process includes high residence times at high temperature.
A further process for isocyanate residue removal comprises using kneader dryers (EP 0 626 368 A1). In this process the abovedescribed heated and stirred containers are replaced by kneader dryers The use of, for example, bitumen has the effect that as in the abovementioned example the remaining residue is obtained as a free-flowing solid which may be employed as a fuel in cement works for example. An advantage of this process compared to the abovementioned process is an increase in yield while the required higher capital expenditure resulting from the more complex technology may be seen as a disadvantage. The use of mechanically moving parts also inevitably results in higher maintenance costs.
EP 0 699 659 A2 describes a process and an apparatus for removing a solid residue from a solution of the residue in vaporizable materials of value and/or solvents by adding up to 20 wt % of high-boiling hydrocarbons inert toward the materials of value under the evaporation conditions and heating the mixture to the evaporation temperature under vacuum, wherein the materials of value evaporate and are drawn off and condensed and the residue is obtained as a free-flowing solid, wherein the residue solution is applied to a stirred bed of granular, solid material which is kept at evaporation temperature. The disadvantage of this process is the additional use of high-boiling solvents which need to be worked up in a further process.
Hydrolysis of isocyanate distillation residues with water to achieve recovery of the starting amine, in particular in the production of TDI, is a field that has been worked on for a comparatively long time already and is described in U.S. Pat. No. 3,128,310, U.S. Pat. No. 3,331,876, GB 795,639, DE 27 03 313 A1 and EP 1 935 877 A1 for example. The cited processes comprise hydrolyzing isocyanate distillation residue with water at elevated pressure and elevated temperature. This converts a portion of the residue into the starting amine which after appropriate workup may be fed back into the phosgenation process thus resulting in residue minimization. What is unsatisfactory in these processes is that a portion of the isocyanate product of value needs to be hydrolyzed back to the starting material and phosgenated again. While this does send the isocyanate present in the residue for useful material recycling it would be desirable to be able to recover from the residue the isocyanate as such.
WO 2007/007887 of Mitsui Chemicals Polyurethanes, Inc. discloses an isocyanate crude product workup which describes not only distillative isocyanate purification but also a two-stage residue concentration. The residue-containing mixture may then optionally be subjected to a hydrolysis reaction which permits recovery of the starting amine. This comprises sending the isocyanate crude product freed of solvent to a distillation column, isocyanate being distilled off under reduced pressure and elevated temperature and residue-containing bottoms product being discharged. This bottoms product has a preferred residue content of 10-40 wt % based on the isocyanate/residue mixture and is conveyed into the second stage of the residue concentration using a pump. This second stage is composed, for example, of a thin-film evaporator which is operated at reduced pressure and comprises an internal condenser. In this evaporator isocyanate is removed, condensed and discharged, an isocyanate-containing residue fraction being transferred to a further processing operation, for example a residue hydrolysis, via a pump. The second concentration stage enriches the residue content to preferably 45-80 wt %, the chlorine content of this fraction being preferably not more than 1.5 wt %, corresponding to 15 000 ppm. Cited as advantages of the described process are the removal of volatile chlorine compounds even in the first concentration stage and the short residence time in the second stage which aims to suppress the continuing viscosity increase through thermally induced polymerization. A disadvantage of such a procedure is that the reported chlorine contents of >1 wt % absolutely still promote thermally induced polymerization in the second stage (which is not operated in pressure- and temperature-optimized fashion) thus leading to not insignificant thermal residue formation which is accompanied by a loss of material of value (TDI). The further workup by residue hydrolysis which follows the residue concentration appears uneconomical and inconvenient for the reported concentrations since while only the still present residual content of material of value (TDI) may be converted back into starting amine by hydrolysis the entire obtaining residue needs to be subjected to the hydrolysis which in turn necessitates a corresponding workup.
DE 102 60 092 relates to a process for purifying crude isocyanate streams in which residue-containing streams are removed in two different steps. To this end the crude isocyanate stream is initially resolved into a residue-containing stream and a gaseous stream in evaporation. While the residue-containing stream is further freed of isocyanate product in a kneader dryer or paddle dryer the gaseous stream is subjected to distillative separation to afford three substreams consisting essentially of low-boiler components, isocyanate product and a further residue-containing stream. The vapor stream from the dryer stage consisting largely of isocyanate product is sent to said distillative separation together with the gaseous stream from the first evaporation optionally after condensation. One disadvantage of this process is that it requires that residue streams be withdrawn and sent for recycling at two different points of the workup.
DE 102 60 093 describes a process for removing isocyanates from a reaction mixture, wherein the reaction mixture freed of the solvent is separated into three fractions in a single separation stage. This affords a tops product composed predominantly of hydrogen chloride and phosgene which is sent for destruction. At the side draw of the column TDI still comprising chlorinated byproducts is withdrawn. TDI and TDI-containing high boilers accumulate at the bottom of the column. In a downstream evaporation crude TDI is again obtained from this high boiler fraction and sent to the TDI obtaining from the side draw. The tar-like residue obtained in the bottoms is likewise sent for incineration. However this document neither describes the residue concentrations achieved in the evaporative concentration nor mentions a concentration of detectable NCO content in the tar-like residue. It further fails to mention that the obtained crude TDI (from bottoms and side draw) is subjected to further purification steps which is absolutely necessary due to the very high content of chlorinated byproducts and which again has a negative influence on the yield balance. It is thus moot whether the described process is a process having economic/large-industrial-scale practicability.
EP 1 413 571 A1 and EP 1 371 633 A1 are concerned with optimizing the workup of TDI by employing a dividing wall column in the distillation which results, inter alia, in a reduction in the content of TDI in the bottoms product. However, accumulation of an isocyanate-containing distillation residue cannot be prevented here either.